1. Field of the Invention
This invention relates to analog-to-digital converters. More particularly, this invention relates to electronic converter circuitry which is adapted to provide very accurate conversions and which is particularly suited for economical manufacture utilizing integrated-circuit (IC) techniques.
2. Description of the Prior Art
Analog-to-digital converters of various types have been in use for many years, for exmple, to convert analog measurements and the like into corresponding digital signals appropriate for processing by high-speed digital computers, for activating digital display devices, and so on. Converters of the so-called successive approximation type have found extensive use, particularly in interfacing with digital computers. There also has been considerable use of converters using electronic ramp-signal integrators with clockpulse timing devices for producing a digital count corresponding to the magnitude of an analog signal. In one such integrator-type converter, sometimes referred to as a single-ramp converter, a known reference voltage is integrated, while a counter counts clock pulses, until the integrator output equals the analog signal; the number of counts is proportional to the ratio of the analog signal to the known reference voltage, and the analog signal thus can readily be determined.
There are still other integrator-type converters which carry out multiple (successive) integration ramps during each conversion. In one converter, as shown for example in U.S. Pat. No. 3,051,939, the unknown analog signal is connected continuously to the integrator input, and a known reference signal of opposite polarity is connected intermittently to the integrator input so as to produce a saw-tooth wave (i.e. ramp-up, ramp-down) at the integrator output; by properly controlling the application of the reference signal, the ratio of the ramp-up to ramp-down times can be used to determine the magnitude of the unknown analog signal from the known reference signal.
In another such converter, as shown for example in U.S. Pat. No. 3,316,547, the unknown analog signal is applied, without any reference signal, to the integrator input and the integrator is activated for a fixed time determined by operating a clock counter to full-scale; the analog signal then is disconnected from the integrator input and replaced with a reference signal of opposite polarity to ramp the integrator back to the zero or start level; the counter reading when the zero level is reached indicates the time required to return to the zero level and thereby represents the ratio of the unknown analog signal to the reference signal. Still another multi-ramp converter, shown in U.S. Pat. No. 3,678,506, operates through three successive ramp-slope phases, so as to obtain a particular ramp-rate when passing through the zero level at the end of the conversion cycle.
There are still other types of converters in general use. For additional information, reference may be made to "Electronic Analog/Digital Conversions," by H. A. Schmid (Van Nostrand Reinhold, 1970).
Prior analog-to-digital converters are known to have a variety of significant disadvantages. For example, converters with relatively high accuracy are too costly for many applications. Other less-costly converters provide inferior performance capabilities, particularly error drift with changes in ambient temperature. Certain converter designs also are inappropriate for integrated-circuit manufacture, in part because they require substantial proportions of certain analog-type circuitry which cannot be produced so readily in IC chip format as can digital-type circuitry. Typical commercially-available converters also are not well suited for handling bi-polar input signals because they require the integrator to be able to ramp both in the positive as well as the negative direction with respect to the start level, depending upon the polarity of the analog input signal; this discontinuity at zero level tends to create additional errors and involves the use of special circuitry adding to the cost of the converter.